Niobium carbide (NbC) and other metal carbides of Group IV, Group V, and Group VI metals are generally ceramic like materials exhibiting extreme hardness. Some of these metals including niobium carbide are useful as or for abrasives, dies, steel additives, ceramic bodies and the like. Niobium carbide is also useful for tool bits.
In the past, NbC which is exemplary of the metal carbides formed according to this invention, has been made by such methods as reactions of niobium oxides or niobium metal with carburizing agents such as elemental carbon or hydrocarbon-containing gases. For the reaction to proceed from these starting materials, extreme reaction temperatures are normally required. Typically, temperatures ranging from 1200.degree.-1700.degree. C. are necessary. These niobium-containing starting materials are expensive to produce and therefore the final carbide product is rendered very expensive. Further, the conventional preparation of niobium oxide and niobium metal has involved the direct chlorination of the niobium containing starting material at high temperatures in special reaction vessels that are graphite lined to reduce the corrosive effect of chlorine gas. If ferroniobium is employed as the starting material, chlorine gas is passed through a bed of ferroniobium at a temperature of from about 500.degree. C. to about 1000.degree. C.
The reaction can be characterized as follows: EQU FeNb+4Cl.sub.2 .fwdarw.FeCl.sub.3 +NbCl.sub.5 +heat
This reaction is exothermic and once started provides considerable heat and must, therefore, be carefully controlled. The FeCl.sub.3 and NbCl.sub.5 produced must be separated and this is accomplished by passing the chlorides in the vapor state through a heated bed of sodium chloride (NaCl) where the FeCl.sub.3 forms a eutectic composition with the NaCl and is thereby removed from the vapor process stream. The NbCl.sub.5 can then be subsequently condensed by cooling.
This chlorination step utilizes toxic chlorine gas reacted exothermically at elevated temperatures and pressures. The conditions produce potentially severe corrosion and safety problems. Special equipment is necessary for handling the highly pressurized, corrosive liquid chlorine and it must be safely vaporized, metered and fed into the reactor. Likewise, the most suitable material for reactor construction is graphite. This is a brittle material which can fracture and fail abruptly after a short time in use in this environment. Further, the chlorine is normally used in excess to ensure complete reaction with the FeNb and the excess must be neutralized creating an expensive, undesirable by-product.
The condensed NbCl.sub.5 can, if desired, be distilled to achieve higher purity material. Distilled or undistilled, NbCl.sub.5 is then hydrolyzed by its addition to water and then the bath is neutralized, and the insoluble product can be dried before being calcined in a heated kiln in the presence of oxygen to produce NbzO.sub.5. The hydrolysis and neutralization steps can produce undesirable by-products and the drying and calcining steps are both energy intensive and expensive.
The Nb.sub.2 O.sub.5 obtained as described can then be metallothermically reduced with aluminum powder in a batch reaction to form Nb metal according to the following equation: EQU 3Nb.sub.2 O.sub.5 +10Al.fwdarw.6Nb+5Al.sub.2 O.sub.3 +heat
This reaction is very exothermic attaining temperatures in excess of the melting point of the products which are then separated by gravity while in the molten state. While expensive, metallothermic reduction is effective with good yields.
The other methods for Nb extraction from FeNb involve caustic or carbonate fusions, which when leached or washed, give niobium oxide which is fairly pure and may be purified further by chlorination or other means presently known to the art. Ultimately, the oxide must be metallothermically reduced as previously described, or carbothermically reduced to Nb metal.
The process of reducing Nb.sub.2 O.sub.5 carbothermically is difficult to do on a production basis since doing so requires large thermal input, vacuum vessels, and a careful balance of carbon to oxygen so that the resulting metal is not contaminated with either carbon or oxygen. If the carbon to oxygen ratio is maintained at nearly stoichiometric amounts, then the reaction proceeds rapidly until only a few percent of either remains unreacted. The reaction then proceeds slowly and it is difficult for it to reach completion. For this reason, carbothermic reduction is not currently used commercially.
Another method for extracting Nb from FeNb could theoretically be the direct electron beam melting and purification of FeNb by preferential vaporization of the Fe. This becomes very expensive in practice as the melting point of FeNb is low and a great amount of electrical power is needed to superheat and vaporize the 20 to 40 weight percent of iron present. Though possible, it is not economically feasible.
Synthesis techniques for niobium carbide, other than reaction with niobium metal, are described in "Refractory Hard Metals" by Schwarzkopf and Kieffer (MacMillan, New York, 1953). Among those cited therein are the reaction of Nb.sub.2 O.sub.3 and mixtures containing niobium oxide including mixtures containing niobium metal, with carbon (as carbon black). This process also requires oxides of niobium which are costly.
Still another reaction scheme described involves reaction of a mixture of niobium chloride, hydrogen and hydrocarbon gases. However, niobium chloride is only available from chlorination of niobium metal or ferroniobium.
It is, therefore, desirable to have a reaction mechanism for obtaining the carbides of the Group IV, V and VI metals utilizing commercially available ferrometal alloys as the starting material, which avoids the complicated costly reactions involving preparation of the metal oxides or metal.